A femoral stem-type prosthesis employing a collar as shown in U.S. Pat. No. 4,404,693 patented Sept. 20, 1983, Karl Zweymuller inventor; U.S. Pat. No. 4,406,023 patented Sept. 27, 1983, William H. Harris inventor; and German Patent No. 26 45 100 patented Apr. 6, 1978, Fritz Hofmeister et al. inventors, are more difficult to remove in the event of post-operative complications than essentially collarless type prostheses such as those described in U.S. Pat. No. 4,310,931 patented Jan. 19, 1982, Maurice E. Muller inventor, since collars reduce access to the fixation surfaces of the femoral stem.
In a collarless type prosthesis such as described by Muller, in the event of need for removal, a thin surgical cutting instrument such as an osteotome or saw blade can be passed superiorly between the fixation surface of the prosthesis and the cortical shell of the proximal femur. This cutting instrument can then be moved inferiorly cutting away the interface between the prosthesis, or in the event cement is used, between the cement and bone or cement and prosthesis, so as to loosen the stem without disturbing significantly the load supporting cortical shell of the femur. Thus, the load supporting cortical shell can essentially be left intact making possible implantation of another stem without difficulty and with the expectation of good results. On the other hand, where a calcar collar is employed the collar generally extends outwardly from the neck and stem of the prosthesis covering the cortical shell of the femur generally anteriorly, medially and posteriorly. With such a collar, it is no longer possible to pass a cutting instrument between the stem of the prosthesis and the cortical shell. As a result, removal of an implanted firmly fixed femoral stem is made more difficult. Generally in cemented prosthesis since the interfacial strength between the prosthesis and cement is relatively weak, the stem can be removed by impacting the stem so as to withdraw it from the femoral cavity. Once the stem is removed, the cement mantle is accessible for removal. Where the stem, however, is fixtured by biological means such as the stem shown in Hofmeister et al. where bone is intended to grow into apertures in the stem, and the porous coated femoral stems now in common use such as the AML.RTM. stem sold by DePuy, a Division of Boehringer Mannheim Corporation, removal of a firmly fixtured femoral stem is often accompanied by substantial loss of the bone of the proximal femur since under such circumstances such impaction will often produce fracture of the cortical shell of the proximal femur and attendant bone loss. Under such circumstances, revision using another stem becomes much more risky and is often attendant by leg length shortening because of this bone loss.
One approach to dealing with this problem is described in U.S. Pat. No. 4,514,865 patented May 7, 1985, William H. Harris inventor, wherein Harris employs a collar contacting cortical bone essentially only along the medial border of the calcar. With such a collar and particularly if there is no porous surface for bone ingrowth on the lateral portion of the femoral stem, removal can be accomplished as described earlier for collarless type prostheses. The disadvantage of this design, however, is that it compromises performance for the overwhelming majority of cases where removal is not necessary in order to provide more convenient removal in a few instances. The load bearing surfaces of the calcar of the most proximal portion of the cortical femoral shell exposed after resection of the neck carries load not only along its lateral border but also its anterior and posterior edges; thus, the somewhat abbreviated collar used by Harris provides less load transfer capability than conventional collar configurations.